It has been known from ancient times that a gentle back-and-forth movement is conducive to the onset of sleep, particularly in infants. Modern research has shown that drowsiness is one symptom of the early stages of motion sickness, which, in its extreme form leads to nausea and eventually emesis. Motion sickness is typically induced as a result of vertical sinusoidal motion, where acceleration is in the range 0.1-0.6 g RMS, and where the frequency is in the range of 9-15 cycles per minute, a typical peak to peak vertical displacement being around 1.5 meters. Obviously, the intentional rocking of a person is meant to induce drowsiness only and no further symptoms, and while experience shows that horizontal vibration alone if continued for an extended period is effective thereto, there is also little doubt that the addition of a small vertical component of motion is beneficial and results in a much reduced requirement for horizontal amplitude and in the desired effect being achieved in a shorter time.
Modern baby cribs are, however, surprisingly deficient with regard to the provision of facilities allowing motion. Some cribs are made without provision for even horizontal movement--attempts to rock baby to sleep are liable to result in the joints of the crib framework loosening or failing. Many cribs are provided with castors; they are an improvement over plain feet, but need considerable continuous effort to achieve the required motion; they tend to scratch the floor and to generate unwanted noise. The time-honoured rocking cradle is hardly in use nowadays, probably because of the danger of instability, though it did offer the important advantage of adding a small vertical component to the acceleration experienced by its occupant, it generated little noise or wear, and once set in motion, would continue to move for a short while even when unattended.